The basic working principle of a dynamometer type instrument is somewhat similar to that of a permanent magnet moving coil instrument. The only difference is that here instead of using permanent magnet we use a current carrying coil for producing the operating magnetic field. The coil used for producing the magnetic field has two halves. We place both the halves side by side. The coil is an air-cored coil. Then, we place a moving coil in between two halves of the fixed field coil. The Dynamometer type instrument is normally spring controlled. We attach a pointer with the moving system.
Torque in Dynamometer Type Instrument
The flux density of the magnetic field (B) produced by the fixed coil is proportional to the current through it. If the current through the field coil is I1, then we can write,
Let us consider, the dimension of the moving coil is L×b, as shown below,
Here, L is the length and b is the breath of the coil.
If the current flowing through the moving coil is I2, we can write the force acting on each side of the moving coil as
Here, N is the number of conductors per coil side.
Hence, we can write the deflecting torque acting the moving coil as,
As a dynamometer type instrument is spring controlled, controlling torque is directly proportional to the angle of deflection of the moving system. We can write,
The deflecting torque equals the controlling torque at the steady position of the pointer. Hence,
So, it is seen that the deflection angle is directly proportional to the product of the currents through the field coil and the moving coil.
Dynamometer Type Ammeter
If we use the instrument as an ammeter, the same current I (say) flows through the field coil and moving coil, and then,
Dynamometer Type Voltmeter
If we use the instrument as a voltmeter, the current flowing through the field coil and moving coil is proportional to the voltage being measured. Ultimately we can write the angle of deflection is also proportional to the square of the voltage to be measured.
Dynamometer Type Wattmeter
We mostly use the dynamometer type instrument as wattmeter. In dynamometer type wattmeter, we use the field coil as the current coil and the moving coil as the voltage coil. The angle of deflection of the pointer is directly proportional to the product of current and voltage. The product of current and voltage is nothing but the power or wattage.
A dynamometer type instrument can be used in AC and DC both because the angle of deflection is proportional to the square of either current or voltage.
Advantages of Dynamometer Type Instrument
- The system is free from hysteresis and eddy current error. Since we use air-core for both moving coil and field coil.
Disadvantage Dynamometer Type Instrument
- The scale of the instrument is not even. Since the angle of deflection of the pointer is proportional to the square of current or voltage to be measured.
- As the magnetic field operates the instrument, we can not utilize eddy current damping in dynamometer type instrument.
- The magnetic field created by an air cored coil is much weaker than that of a permanent magnet. The sensitivity of the instrument is lower than that of other types of measuring instrument.
- To produce required magnetic force at the moving coil in a comparatively week magnetic field, the moving coil must have a large number of turns, which makes the moving coil heavier.
- The instrument is spring controlled. Phosphor bronze springs attached to the moving system make the moving system further heavier. The heavier moving system suffers from frictional error.
- Current in the field coil produces heat and heat increases the resistance of the coil itself, which may cause an error in the reading.
- The external magnetic field can seriously affect the performance of the instrument. So, the instrument requires proper magnetic screening for error-free readings.
- The heavier moving system causes low torque to weight ratio.
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